The present invention concerns a method for linearization of a high-frequency amplifier, to which a predistorter is connected upline in which a predistortion factor is altered adaptively so that a linear relation exists between the output signal of the high-frequency amplifier and the input signal of the predistorter.
The transmission behavior of a high-frequency amplifier is highly dependent on its rest current (bias voltage). The higher the bias current, the better the linearity of the amplifier in general. However, the power consumption of the amplifier increases and the efficiency diminishes with the closed-circuit current. This can lead to a significant cost in power supply, cooling, etc., especially in high-frequency power amplifiers. If high linearity of the amplifier is dispensed with, this can be operated with a lower closed-circuit current so that the cost of power supply and cooling of the amplifier is reduced. Operation of the amplifier in its linear control range can be omitted if a predistorter that largely compensates for the distortions of the amplifier is connected upline of the amplifier. Predistorters for high-frequency amplifiers are known from the literature, for example, IEEE Transactions on Communications, Vol. 45, No. 10, October 1997, pp. 1167-1171. The adaptive digital predistorter described in this article operates as follows. The amount of the signal to be predistorted is multiplied by a real value and a real value added to the phase in order to balance the nonlinear effects of the amplifier. The corresponding values for amount and phase are read from a-table whose address is determined from the amplitude of the signal being predistorted. The amount and phase are adapted separately. The correction value to control adjustment of the amount is the difference in amounts of the preamplifier output signal and the signal being predistorted. The correction value for phase adjustment is the difference in phases of the amplifier output signal and the signal being predistorted.
The task underlying the invention is to provide a method of the type just mentioned with which the best possible linearization of a high-frequency amplifier can be achieved.
The stated task is solved by the features of claim 1, in that the quotient of the output signal of the high-frequency amplifier and the input signal of the predistorter is initially formed and an average amplification then determined from the signal quotient. A correction term is formed for a predistortion factor formed in the predistorter that depends on the deviation of the average amplification relative to a reference value. Finally, the predistortion factor is altered adaptively so that the correction term becomes minimal. According to the invention, the amount and phase of the signal need not be calculated, as in the cited prior art, but instead the quotient of the formed signal, so that the correction term for adjustment can be determined with limited calculation cost.
Since the characteristic of the predistorter is adaptively set with this method, temperature- and aging-dependent amplification changes can also be compensated. An automatic adjustment of the predistorter during operation also makes adjustment of the predistorter during manufacture unnecessary.
Advantageous modifications of the invention are apparent from the subclaims. A particular good convergence during adjustment of the predistorter is obtained by the fact that the correction term for the predistortion factor is formed from the product of an adjustment constant and the difference between the inverse average amplification and 1.
It is expedient that the power or a quantity derived from it be formed from the input signal of the predistorter, that the power or the quantity derived from it then be linearly quantized, that a value of the predistortion factor be assigned to each possible quantization step, that an average amplification be determined for each quantization step, and that for each distortion factor allocated to a quantization step a correction term dependent on the average amplification determined at the corresponding quantization step be formed, which is minimized by adaptive variation of the predistortion factor. The average amplification of each quantization step is advantageously determined by the fact that amplification values are formed over a specified time interval from the time- and value-discrete scanning values of the output signal of the high-frequency amplifier and the input signal of the predistorter, that all amplification values corresponding to a corresponding quantization step are accumulated, and that for each quantization step the amplification value resulting at the end of the time interval from accumulation is divided by the number of accumulation steps.
The adjustment constants preferably have a value dependent on the corresponding quantization step to which the predistortion factor is assigned.
An optimal adjustment rate for the predistortion factor can be achieved by the fact that the smaller of two values is chosen as adjustment constant, in which the one value is 0.5 and the other value depends on the number of accumulations of amplification values in the corresponding quantization step.